MRI magnets, used for diagnostic imaging, usually use superconducting coils to generate the high intensity static magnetic field needed in such systems and some type of a shield to prevent stray field from contaminating spaces where general public has access.
There are two basic methods for shielding the MRI magnet: ferromagnetic shielding and active shielding. The first method involves placing iron around the MRI magnet in form of a return Yoke.
The second method involves using active shielding magnets. The methods for active shielding magnets use counter-running coils with currents in opposite directions and iron in various combinations.
In active shielding for MRI systems two sets of coils are used: a first set of coils, responsible for the main homogenous magnetic field, and a second coil system, which is used to provide the active shielding. Usually the coils are made of superconductive material.
When constructing an active shield magnet prior art has required that the outer and inner magnets are wound on separate formers and the formers themselves are later assembled together. This was because coils overlapped along the Z axis.
In order to assemble the shielding coils, which may be subject to significantly large forces, it is necessary to have massive components to accommodate the fixings. It is an unfortunate consequence of this technique that the accumulation of the smallest tolerances and relative movements of parts of the assembly under load leads to important degradation of homogeneity.
Technically each set of coils was fitted in slots on its own special coil support which is called a former, i.e. the first set of coils 22 responsible for the main homogenous magnetic field is fitted on a first former 23, and the second set of coils 21 responsible for the active shielding is fitted on a second former 24 (FIG. 1). This structure which uses at least two separate formers for an MRI system was disclosed in prior art see for example U.S. Pat. Nos.: 5,329,266, 5,210,512, 5,296,810, 4,890,082, 5,045,826. In particular, Elscint's U.S. Pat. No. 5,012,217 on an integrated active shielded magnet is further improved by this disclosure.
Active shield magnets for use in MRI can be constructed in two principal ways. Either the wire is wound directly onto a former or the wire is wound into a mould and impregnated with resin. The second method produces separate free-standing coils which can be assembled onto a former or formers at a later time. Both methods have been used to construct working magnets.
Winding the wire directly onto a former is today the preferred method by the MRI manufacturer. When winding the wire directly onto the former the process time is shortened, thus saving valuable time. Furthermore the direct winding is more accurate in placing the coils exactly at the right position on the former compared with the moulding-assembly route.
Active shield systems with more than one former, as disclosed in prior art, have many disadvantages. For example it is difficult to assemble the two formers, thus the accuracy of the system is decreased. Another disadvantage of such a system is its size which is usually large. Yet another disadvantage of the two-former system is the fact that its coils can not be wound on the former itself in a single process. This means that the winding process becomes more expensive,, slower, and less accurate!.
One solution offered by prior art to try and solve the difficulty in assembling the two formers was using some type of a single former. For example see U.S. Pat. No. 5,136,273. In this cited art a single former is used and the shielding coils are placed at the ends of the former at a larger radius than the inner main coils. Such a solution as described in the above cited art solves the assembly problem but does not enable winding the coils in a single process nor does it give the most compact solution for a given homogeneity.
Because the coils cannot all be wound directly onto the single former, the possibility of achieving improved accuracy is lost.
Thus, a compact MRI magnet with a single former which allows a single winding process is most desired. Such a system offers great advantages in the construction of the magnet by making the coil winding process faster, cheaper, and easier to control.
It is the intention of the present invention to introduce an improved active shielding MRI system which is more compact, and uses a single former which can be wound in a single process.